A source of benzene and xylene is catalytic reformate, which is prepared by mixing petroleum naphtha with hydrogen and contacting the mixture with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. Usually, a C.sub.6 to C.sub.8 fraction is separated from the reformate, extracted with a solvent selective for aromatics or aliphatics to separate these two kinds of compounds and to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains benzene, toluene and xylenes (BTX), along with ethyl benzene.
Refineries have also focused on the production of benzene and xylene by transalkylation of C.sub.9 + aromatics and toluene over noble metal-containing zeolite catalysts. During the transalkylation of C.sub.9 + aromatics and toluene to high value petrochemical products, such as benzene and xylene, over catalysts containing noble metals, by-product saturate compounds are typically produced during the first several months on stream. These by-product saturate compounds, referred to as coboilers, can boil in the same temperature range as a high value petrochemical product, making separation of the high value petrochemical product at high purity levels difficult. For example, a benzene product for commercial sale must exceed 99.85% purity. However, initial benzene product purity after distillation of a transalkylation reaction product is typically only about 99.2% to 99.5% due to the presence of coboilers, such as methylcyclopentane, cyclohexane, 2,3-dimethylpentane, dimethylcyclopentane and 3-methylhexane. Therefore, an additional extraction step is usually required to further improve benzene product purity to the desired level.
In view of the difficulty in obtaining a high purity benzene petrochemical product due to the presence of coboilers that are formed during the transalkylation of C.sub.9 + aromatics and toluene over noble metal-containing zeolite catalysts, it is desirable to reduce the level of coboilers that is produced in the transalkylation reaction. An advantage of reducing the level of coboilers that is produced in the transalkylation reaction is that a high purity benzene product may be obtained after distillation of the transalkylation reaction product, without the need for an additional extraction step, thereby reducing the number of steps that is required to obtain a benzene product having a purity of at least 99.85%.